Add a another page flag that can be used to figure out if a compoundpage is virtually mapped. The mark is necessary since we have to knowwhen freeing pages if we have to destroy a virtual mapping.

No additional flag is needed. We use PG_swapcache together with PG_compound(similar to PageHead() and PageTail()) to signal that a compoundpage is virtually mapped. PG_swapcache is not used at this point sincecompound pages cannot be put onto the LRU (yet).